Carbon nanotubes, like fullerenes, are comprised of shells of carbon atoms forming a network of hexagonal structures, which arrange themselves helically into a three-dimensional cylindrical shape. The helix arrangement, or helicity, is the arrangement of the carbon hexagonal rings with respect to a defined axis of a tube. Generally, the diameter of a nanotube may range from approximately 1 nanometer (“nm”) to more than 100 nm. The length of a nanotube may potentially be millions of times greater than its diameter. Carbon nanotubes are chemically inert, thermally stable, highly strong, lightweight, flexible and electrically conductive, and may have greater strength than any other known material.
Common methods for the manufacturing of nanotubes include high-pressure carbon monoxide processes, pulsed laser vaporization processes and arc discharge processes. These processes produce nanotubes by depositing free carbon atoms onto a surface at high temperature and/or pressure in the presence of metal catalyst particles. The nanotubes typically form as bundles of tubes embedded in a matrix of contaminating material composed of amorphous carbon, metal catalyst particles, organic impurities and various fullerenes depending on the type of process used. Bundles of nanotubes formed by these manufacturing methods can be usually extremely difficult to separate.
Current methods for purifying and isolating nanotubes to remove contaminating matrix surrounding the tubes employ a variety of physical and chemical treatments. The treatments include high temperature acid reflux of raw material in an attempt to chemically degrade contaminating metal catalyst particles and amorphous carbon, the use of magnetic separation techniques to remove metal particles, the use of differential centrifugation for separating the nanotubes from the contaminating material, the use of physical sizing meshes (i.e., size exclusion columns) to remove contaminating material and physical disruption of the raw material utilizing sonication. Additionally, techniques have been developed to partially disperse nanotubes in organic solvents in an attempt to purify and isolate the structures. The uniformity of a matrix may also be improved by lowering the amount of nanotubes, however the overall composite strength is proportionally reduced.
The use of carbon nanotubes has been proposed for numerous commercial applications, such as, for example, catalyst supports in heterogeneous catalysis, high strength engineering fibers, sensory devices and molecular wires for electronics devices. Accordingly, there has been an increasing demand for carbon nanotube structures that are free of impurities which often occur due to defects and variations in production, or growth rate. Additionally, although individual Carbon nanotubes have demonstrated useful properties when used as a filler in composite materials, those aggregate properties fall short of what would be expected. This is due in part to the presence of defects and variations, the tendency to bundle which prevents full or uniform dispersal in a composite, and the common interference/attractive effects between individual isolated nanotubes.
It would be advantageous to provide carbon nanotubes which overcome the above shortcomings.